Calculator
Example Data Table
| Fraction | Total activity, U | Protein, mg | Specific activity, U/mg | Purification fold |
|---|---|---|---|---|
| Crude extract | 250 | 50 | 5 | 1 |
| Salt cut | 190 | 18 | 10.56 | 2.11 |
| Column fraction | 125 | 5 | 25 | 5 |
Formula Used
Specific activity = Total enzyme activity ÷ Total protein mass.
Specific activity = U ÷ mg.
Product method: Activity concentration = (µmol product ÷ time ÷ enzyme assay volume) × dilution factor.
Absorbance method: Reaction rate = Corrected ΔA/min × reaction volume ÷ extinction coefficient ÷ path length.
Total activity = Activity concentration × total enzyme stock volume.
Yield = Total activity ÷ starting total activity × 100.
Purification fold = current specific activity ÷ initial specific activity.
Estimated kcat = specific activity × molecular weight in kDa ÷ active fraction.
How to Use This Calculator
Select the method that matches your assay data.
Use direct activity when total units are already known.
Use product mode when product amount and reaction time are known.
Use absorbance mode when your assay gives a slope.
Enter the protein mass directly, or enter protein concentration and volume.
Add dilution factor when the assayed enzyme was diluted.
Enter optional reference values for yield and purification fold.
Press calculate to show the result above the form.
Use the CSV or PDF buttons to save the same result.
Specific Activity in Enzyme Work
Specific activity shows how much catalytic power exists in each milligram of protein. It is a compact quality number for enzyme extracts, purified fractions, and stored samples. A higher value usually means the preparation contains more active enzyme per protein mass. It does not always prove purity, but it is very useful during purification.
Why This Value Matters
Laboratory teams track specific activity across crude lysate, precipitation, chromatography, and final concentrate steps. Each step should remove inactive protein while retaining useful activity. When activity rises per milligram, purification is working. When it drops, the enzyme may be denatured, diluted, inhibited, or measured under poor assay conditions.
Main Measurements Needed
The calculation needs enzyme activity and protein mass. Activity is often expressed in units. One unit means one micromole of product formed per minute under stated conditions. Protein mass may come from a Bradford, BCA, Lowry, absorbance, or direct concentration reading. Always keep assay temperature, pH, substrate level, and incubation time consistent.
Using Absorbance Rates
Many assays measure change in absorbance each minute. Beer Lambert law converts that slope into product formation rate. The extinction coefficient, path length, and reaction volume must match the product signal. Dilution factor also matters. A missed dilution can change the answer by tenfold or more.
Interpreting Results
Specific activity is reported as units per milligram. Compare fractions from the same method, not from unrelated conditions. If a reference value is available, percent reference activity helps show relative performance. Optional turnover estimates can be calculated when molecular weight and active fraction are known. Treat those estimates carefully because impure enzyme changes them.
Good Laboratory Practice
Use blank corrected absorbance. Run replicates when possible. Use the linear part of the reaction curve. Avoid substrate depletion and long incubations that bend the rate. Record all volumes and dilution steps before calculating. This calculator keeps the arithmetic transparent, supports several input paths, and lets you export results for notebooks, reports, or quality records.
Common Sources of Error
Pipetting small enzyme volumes can create large variation. Old substrate, cloudy samples, and dirty cuvettes also distort rates. Calibrate instruments often. Label fractions clearly. Recalculate after every dilution. Store raw readings with every final value for review.
FAQs
What is enzyme specific activity?
It is enzyme activity divided by protein mass. The usual unit is U/mg. It shows catalytic activity per milligram of protein in the tested preparation.
What does one enzyme unit mean?
One unit usually means one micromole of product formed per minute. This definition depends on stated assay conditions, including pH, temperature, substrate, and method.
Can I use absorbance data?
Yes. Enter the absorbance change per minute, blank rate, extinction coefficient, path length, and reaction volume. The calculator converts the slope into product formation rate.
Which protein value should I enter?
Enter total protein mass if known. Otherwise, enter protein concentration and protein volume. The calculator multiplies those values to estimate protein mass.
Why is dilution factor important?
Dilution factor corrects the assayed activity back to the original enzyme preparation. Missing it can make the result too low, sometimes by a large amount.
Does high specific activity prove purity?
Not alone. It often suggests enrichment, but inhibitors, assay conditions, and enzyme damage can affect the value. Use it with gels, chromatography data, or other checks.
What is purification fold?
Purification fold compares current specific activity with the initial specific activity. A value of five means the enzyme is five times more active per milligram than before.
Can this estimate kcat?
Yes, when molecular weight and active fraction are entered. The estimate assumes the measured protein is active enzyme, so impure samples can give misleading values.